Tabs are not allowed

[apparentlymart]

This repo inherited a restriction from an early prototype that tabs are not allowed.

This was done as part of an experiment that didn't end up working out, so this restriction should be removed and instead the scanner should permit tabs as a funny sort of space, counting them as a single "column" for source-location-reference purposes.

The canonical style is still to indent with pairs of spaces, so the auto-formatter in hclwrite will continue to replace tabs with spaces.